Plastic foams are well known and are typified as being 2-phase gas-solid systems in which the solid is continuous and composed of a synthetic polymer. The gas phase is usually distributed in voids or pockets called cells which may be interconnected in such a manner that the gas can pass from one cell to another or are discrete such that the gas phase of each cell is independent of that of the other cells. Generally, foamed plastics are achieved by dispersing an inert gas through the polymer melt either by direct introduction into the melt or by pre-blending with a chemical blowing agent that decomposes with heat. Exemplary of some of the plastics which have been foamed include polystyrenes, polyolefins, polycarbonates, polyurethanes, polyesters, and aldehyde based condensation resins and the like, U.S. Pat. Nos. 2,959,508; 3,717,559; 3,704,269; 3,753,933; 3,863,000; 4,097,425; 4,002,581; 4,226,949; 4,247,652 and 4,312,960, generally. A preferred class of plastic foams are those known as structural foams. In general, structural foam plastics can be defined as products made from virtually any high-molecular-weight thermoplastic organic polymer having a cellular core with an integral skin on all sides. The skin is relatively non-porous in relation to the cellular core. They tend to have a high strength-to-weight ratio and a density of greater than about 0.3 grams per cubic centimeter.
While most organic polymers have been foamed, others have not been successfully foamed due to the lack of or inability to find suitable blowing agents. This is especially true for low modulus thermoplastics, particularly copolyetherester resins and blends thereof with other high-molecular weight thermoplastic polymers. Recent attempts to foam low modulus thermoplastics have resulted in parts manifesting very irregular, non-uniform internal cellular structures. Their surface is characterized by sink marks and rough surface texture as a result of the collapse of the cells. Further, they suffer from very uneven external skin, wherein certain portions are disproportionately thicker or thinner than others. Finally, without the aid of nucleating agents, these compositions have very little cellular development at all and what cellular development there is, is very non-uniform.
Therefore, it is an object of the present invention to identify suitable blowing agents for copolyetheresters wherein said blowing agents generate gas as a consequence of their physical change or decomposition at a temperature at or near the processing temperature of the polymer to be foamed and said gases and/or other decomposition products do not deleteriously affect the polymer resin.
It is also an object of the present invention to prepare low modulus engineering thermoplastic structural foams wherein the low modulus thermoplastic polymer is especially copolyetheresters or blends thereof with other high-molecular weight thermoplastics.